The less well-known but extremely useful (and standard since C89 — meaning 'forever') functions in the C library provide the information in a single call. Actually, there are multiple functions — an embarrassment of riches. The relevant ones for this are:

7.21.5.3 The strcspn function

Synopsis

#include <string.h>
size_t strcspn(const char *s1, const char *s2);

Description

The strcspn function computes the length of the maximum initial segment of the string
pointed to by s1 which consists entirely of characters not from the string pointed to by
s2.

Returns

The strcspn function returns the length of the segment.

7.21.5.4 The strpbrk function

Synopsis

#include <string.h>
char *strpbrk(const char *s1, const char *s2);

Description

The strpbrk function locates the first occurrence in the string pointed to by s1 of any
character from the string pointed to by s2.

Returns

The strpbrk function returns a pointer to the character, or a null pointer if no character
from s2 occurs in s1.

The question asks about 'for each char in string ... if it is in list of invalid chars'.

which, if it did, then strchr would indeed be the most suitable answer. If, however, there is no null termination to an array of chars or if the chars are in a list structure, then you will need to either create a null-terminated string and use strchr or manually iterate over the elements in the collection, checking each in turn. If the collection is small, then a linear search will be fine. A large collection may need a more suitable structure to improve the search times - a sorted array or a balanced binary tree for example.